Working model for prosthodontic preparation of a crown for installation on an implant fixture

ABSTRACT

A method of producing a working model for preparation of a clinical crown for attachment to an implant fixture in the mouth of a patient is described in which a laboratory analog, on which the crown is mounted during preparation, is formed as an assembly including a fixed sleeve and a detachable pin and wherein the crown-receiving portion of the pin element of the assembly replicates the crown-receiving portion of the implant fixture. The analog sleeve is fixedly mounted in a base segment of the model so that the analog pin bearing a crown to be prepared threadedly connected thereto can be removed from and returned to mounted installation in the model by simple sliding movement between the pin and sleeve. Also described is the resultant working model construction and the particular analog elements which also form parts of the invention.

BACKGROUND OF THE INVENTION

The present invention relates to the laboratory preparation of dentalprostheses for installation on implant fixtures previously installed inthe mouth of the patient. More particularly, the invention relates tothe construction of a working model and the analog elements employedtherein for use by a dental technician in the fabrication of restorativedental prostheses.

Dental prostheses which are fixedly attached to the jaw of a patientthrough the use of dental implants are common. Such prostheses employimplant fixtures which are permanently fused to the jaw of the patient,and crowns comprising abutments, upon which a prostheses is formed, forultimate attachment to the implant fixture. Typically, a two-stageprocedure is employed. In the first stage, an implant fixture isinstalled in a receptacle formed in the bone of the jaw of the patentand the fixture allowed to become bonded to the bone by osseointegrationoccurring over a period of several months. During the second stage ofthe procedure, the gum that overlies the now-fused implant fixture isincised to expose the top end of the fixture and a previously preparedcrown is attached thereto by fitting the crown abutment to anabutment-receiving portion of the fixture and connecting the crown bymeans of a suitable screw that cooperates with a threaded socket in theimplant fixture.

The description herein is directed principally to prostheses of theso-called "UCLA"-type in which the abutment is integrally-formed withthe prosthesis casting. It should be understood, however, that theinvention is equally applicable to standard abutment-cylinder assembliesin which the abutment is a separately formed element designed forthreaded attachment to the fixture, as hereinafter described.

In preparing a dental prosthesis or crown for attachment to an implantfixture, a work holder, called a "working model", in the form of anaccurate replica of the existing tooth and gum structure of the patientis made from molding material, such as die stone or plaster-of-paris.Such working model is utilized by the technician during the constructionof the crown, which normally comprises a hard core or "abutment" thatmay be formed of metal, ceramic, plastic, or other hard material, andcovered by a ceramic material, usually porcelain, or by plastic orcomposite materials that replicate the tooth or tooth group to bereplaced. (In the description herein, the crown consists of two lowercentral teeth. The invention, however, contemplates crowns for replacinga single tooth, or groups of teeth, for the described or other regionsof a patient's mouth.)

In essence, the working model is constructed by making an impression ofthe teeth and gums in the concerned region of the patient's mouth byenclosing them in dental impression material, as for example, silicone,or other formable rubber-like materials, to form a mold or "negative"into which molding material, such as plaster-of-paris, is placed andpermitted to harden. The solidified model forms a work holder, sometimestermed, "a replica cast", which duplicates the affected region of thepatient's dentition and within which the crown may be inserted duringpreparation in order to accurately determine its relationship toadjacent teeth for fitting purposes.

Presently, it is the practice in producing working models of theconcerned type to make a single casting of molding material duplicatingthe patient's dentition and in which a laboratory analog is fixedlyinstalled at the proper location. The upper end of the analog containsan internally threaded socket and an abutment-receiving portion that, incooperation with the socket, enables a crown intended for ultimateattachment to an implant fixture to be set in the working model.Attachment of the crown to the laboratory analog is effected by passinga headed screw through an open-ended axial hole extending through thecrown abutment and connecting it to an internally threaded socket in theanalog. Typically, the screw employed for connecting the crown to theanalog contains a slotted head for reception of a screw driving tool andis structurally similar to the screw employed for connecting the crownto the patient's implant fixture.

Thus, in preparing the crown, during the process of which the crown mustbe repeatedly removed and re-installed in place in the working model, itis necessary that the technician employ the screw driving tool torelease the screw upon each removal of the crown from the working modeland, conversely, to install the screw upon each mounting of the crown inplace in the working model. This repeated manipulation of the screw eachtime the crown is removed or reinstalled on the working model islaborious and time-consuming and consequently adds substantially to thetime and effort required for preparation of the crown. Furthermore, itincreases the danger of eroding or otherwise damaging the cast workingmodel.

It is, therefore, to the solution of this problem that the presentinvention is directed.

SUMMARY OF THE INVENTION

According to one aspect of the invention, therefore, there is provided amethod for constructing a working model apparatus for use by alaboratory technician in the preparation of a prosthetic crownparticularly intended for ultimate attachment to an implant fixturepreviously installed in the mouth of a patient comprising the steps ofattaching a coping to the implant fixture mounted in the mouth of thepatient; pouring dental impression material about the selected region ofthe mouth of the patient to enclose the coping and the region of themouth on adjacent sides thereof to form an impression mold; removing anassemblage of the formed impression mold with the coping attached fromthe patient's mouth and connecting the assemblage to an impression tray;attaching to the exposed end of the coping an assembled separableimplant fixture analog pin and sleeve assembly; packing molding materialin the formed impression to a predetermined level leaving the analogsleeve exposed; permitting the mold material to solidify to produce aremovable model segment; thereafter, placing additional molding materialin the formed impression to substantially cover the exposed surface ofthe solidified removable model segment and the analog sleeve; permittingthe additional molding material to solidify to produce a stationarymodel segment; removing the impression mold and the coping; andattaching a clinical crown to the pin element of the pin and sleeveassembly.

There is also provided by the invention, a working model apparatus ofthe heretofore described type comprising a stationary segment and amovable segment, both formed of a solidified molding material; thesegments being connected for separation at a parting line therebetween;an analog sleeve containing an axial base fixedly secured on thestationary segment; an analog pin axially slidably received in the boreof the analog sleeve, the pin having an external shape adapted for snug,slidable reception in the sleeve bore and its upper end being formed toreplicate a crown abutment-receiving portion of the implant fixture; andcooperable means on the pin and sleeve to prevent relative rotationalmovement between the pin and sleeve in their assembled condition.

The invention further contemplates a laboratory analog for installationin a working model utilized by a technician in the preparation of acrown intended for ultimate attachment to an implant fixture comprisinga fixture analog sleeve having an external surface configured for lockedretention in a body of solidified molding material in anaxially-extending material bore; an analog pin having an external shapefor snug, sliding reception to a predetermined extent in the bore of theanalog sleeve; cooperating elements on the analog sleeve and the analogpin to prevent relative rotation therebetween; and an abutment-receivingportion on an exposed, crown-receiving end of the pin replicating thecrown-receiving portion of the installed implant fixture, and means inthe crown-receiving portion for attaching a clinical crown thereto, themeans substantially replicating the means for attaching the crown to theimplant fixture.

Pin and sleeve assemblies for use in working models in the preparationof dental prostheses are not new. Typical examples of prior artassemblies of this type are shown in U.S. Pat. Nos. 3,518,761,4,054,995, 4,363,625, 4,801,264, and 4,955,811, in only the last ofwhich is preparation of a crown for ultimate attachment to a previouslyinstalled implant fixture contemplated. Furthermore, nowhere in theprior art is there shown a method of constructing a working model inwhich an analog assembly installed in the model contains a pin whoseabutment-receiving portion replicates the abutment-receiving portion ofthe implant fixture for which the crown is being prepared. Consequently,only from the present invention is there obtained the aforementionedbenefits of rapid and effortless manipulation to and from a workingmodel of a crown by a technician and of the avoidance of damage orerosion of the model during preparation of the crown prior to attachmentto an implant fixture.

For a better understanding of the invention, its operating advantages,and the specific objections obtained by its use, reference should bemade to the accompanying drawings and description which related to apreferred embodiment thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a lower central prostheticcrown of the UCLA-type clinically attached to an implant fixtureinstalled in the mouth of a patient;

FIG. 2 is a schematic illustration of a prior art working model andlaboratory analog for producing the prosthetic crown of FIG. 1;

FIG. 3 is a schematic illustration of a working model and analogassembly constructed in accordance with the present invention forproducing the prosthetic crown of FIG. 1;

FIG. 4 is a schematic illustration of the initial step in theconstruction of the described working model involving the molding of adental impression in the mouth of a patient;

FIG. 5 is an enlarged partial sectional view of the step illustrated inFIG. 4 and showing an implant impression coping attached to an installedimplant fixture;

FIG. 6 is a view similar to FIG. 5 but showing a coping attached to animplant analog assembly in a working model according to the presentinvention;

FIG. 7 is an enlarged perspective view of a typical laboratory implantanalog assembly according to the present invention;

FIG. 8 is a sectional elevation view of the implant analog assemblyshown in FIG. 7;

FIGS. 9, 10, and 11 illustrate an impression tray in progressive stagesof working model production, FIG. 9 showing the impression tray with theformed impression received therein, FIG. 10 showing solidified die stoneand "soft tissue" layer formed within the impression, and FIG. 11showing a solidified layer of dental stone superimposed on the diestone;

FIGS. 12 to 29 are illustrations showing, in sequence, a partialprospective representation of the abutment-receiving portion of animplant fixture, a partial elevational view, and a partial perspectiverepresentation of a replicated laboratory implant analog assemblyaccording to the invention for several embodiments of implant analogassemblies contemplated by the invention;

FIG. 30 is a partial sectional representation of a crown prostheses ofthe standard abutment and cylinder type installed on an implant fixture;and

FIG. 31 is an illustration similar to FIG. 30 showing an implant analogassembly according to the invention for use with the crown prostheses ofFIG. 30.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 of the drawing illustrates a fixed dental prosthesis in the formof a crown 10 mounted on an implant fixture 12 that has been fixed inthe bone 14 of a patient's jaw by the medium of osseointegration. Forpurpose of description, the illustrated prosthesis is of the UCLA-typeand consists of the two central teeth 16 and 18 of the lower jaw. Thecrown 10 comprises a core and integrally formed abutment 20, typicallyformed of metal, usually gold, upon which is deposited a ceramic facing21, typically porcelain, so-formed as to replicate the dentition to berestored. The abutment 20 contains a bore 22 adapted to receive theshank of a headed screw 24. The upper portion 26 of the bore 22 is ofenlarged diameter to receive the head of the screw 24 and cooperateswith the bore to define a shoulder 28 for seating the screw head. Thelower portion of the bore is formed as a recess 30 complementary to theabutment-receiving portion of the implant fixture 12, which is typicallyformed as a headed cylinder having a threaded exterior to facilitateinstallation in a jaw bone and to assist in preventing retrogrademovement of the implant fixture upon completion of osseointegration.

A socket 32 extends axially into the implant fixture 12 and is threadedfor reception of the screw 24. In the illustrated device, theabutment-receiving portion of the implant fixture 12 is identifiedgenerally as 34 and is particularly structured, in cooperation with therecess 30, to seat the crown 10 in a manner to prevent rotationalmovement between it and the fixture. As shown, the abutment-receivingportion 34 of the implant fixture 12 contains a raisedhexoganally-shaped boss 36, which is illustrated in greater detail inFIG. 12 and is intended for reception in the complementarily-shapedrecess 30. It should be understood, however, that the instant inventionis not limited to a particular abutment-receiving portion of an implantfixture, but, instead, as explained in more detail hereinafter,contemplates any and all of the currently known abutment-mountingconfigurations for implant fixtures suitable for use with the invention,as by the way of example but not by way of limitation, are shown inFIGS. 12 and 31 herein.

In restorative dentistry of the concerned type, it is necessary that anexact replica be made of the region of the patient's mouth in which theimplant fixture 12 is installed and in which the crown 10 is to bemounted. Such replica is known as a "model" or "working model" and istypically formed of plaster-of-paris or die stone molded in a "negative"created by an impression taken of the concerned dentition in which thedentition is impressed in the impression material, that may comprisesilicone rubber, or an equivalent moldable material, all as is generallyknown in the dental arts.

FIG. 2 of the drawings shows a typical prior art model arrangement inwhich an implant laboratory analog 38 is embedded in a model 40 formedof plaster or die stone. A resilient layer formed of silicone-typematerial, is usually applied about the top of the analog 38 to replicatesoft gum tissue. In order to form the model 40 according to prior artpractice, an implant impression coping 44 is, as shown in FIGS. 4 and 5,attached in the patient's mouth to the previously installed implantfixture 12. The coping 44, which has at its lower end a complementaryrecess 46 to engage the abutment-receiving portion 34 of the implantfixture 12, is provided with a hollow interior for reception of anelongated coping screw 48 whose lower end is threaded and of a diameterto permit engagement with the threads in socket 32. The exterior surfaceof the coping 44 is preferably provided with means, here shown asopposed flats 50, to facilitate retention of the impression material 52into which the patient's dentition, including the coping, is immersed(see FIG. 4) to make the "negative". Following set-up of the impressionmaterial, the formed impression 52 is invertedly inserted in animpression tray 54 with the coping 44 in place and the threads of thecoping screw 48 exposed for reception of an implant analog 38 that isattached to the screw by means of engagement with the threaded socket 56of the analog shown in FIG. 2.

Following the attachment of the analog 38 to the coping screw 48 in theimpression 52, the impression is filled with a highly viscous moldingmaterial, in the form of moldable plaster or die stone, which, whenhardened, creates the model arrangement shown in FIG. 2 wherein theimplant analog 38, except for its upper surface 58, which replicates theabutment-receiving portion of the patient's implant fixture 12, isembedded. A soft tissue layer 60 having a through-hole 62 exposes thesocket 56 in the analog upper surface 58. Thus, as shown in FIG. 2, thecrown 10 is capable of being secured to model 40 by attachment to theexposed upper surface 58 of analog 38 by means of connection of a screw,identical to the implant screw 24, with the analog socket 56. Access forthe screw 24 to the socket 56 is provided by the bore 22 in the crownabutment 20 and an opening 64 in the unfinished porcelain facing 21.

It will be appreciated that, in order for the laboratory technician toprepare the crown 10, which involves repeated installations and removalsthereof to and from the model 40, the technician through the use of atool, such as a screw driver (not shown), will either install or removethe screw 24, as the case may be.

When preparation of the crown 10 has been completed, an incision is madeby the dentist in the gum tissue overlying the implant fixture 12 in thepatient's mouth and the crown is attached to the fixture by threadedengagement of a screw 24 with the fixture socket 32. Finally, theopening 64 in the facing 21 is filled with porcelain filling material,or the like, to complete the restoration procedure.

The present invention provides an improved practice for the fabricationof a working model and an improved form of implant analog device for usein the working model and particularly adapted for the preparation ofcrowns intended for attachment to implant fixtures. According to theinvention, the implant analog device of the prior art, such as thatshown at 38 in FIG. 2, is replaced by an analog assembly identifiedgenerally by reference numeral 70, the details of which are best shownin FIGS. 7 and 8. As shown in these figures the analog assembly 70comprises an elongated open-ended sleeve 72 which has conically taperedinterior and exterior surfaces, 74 and 76 respectively. The lower, smalldiameter end of the sleeve contains an annular flange 78 concentric withthe bottom opening 80 and operative to assist in preventing axialretrograde movement of the sleeve from the model. Similarly, theexterior surface 76 of the sleeve is provided intermediate its ends witha chordal flat 82 operative to prevent rotational movement of the sleevewithin the model.

An analog pin 84 having a generally tapered exterior surface 86 isdimensioned to be snugly, slidingly received in the interior of thesleeve 72. The pin 82 is slightly longer in axial length than the sleeve72 such that, when the pin is fully inserted in the sleeve, the lowerend of the pin extends through the bottom opening 80 and beyond the endof the flange 78, as shown in FIG. 8. The surface of the pin 84,adjacent its lower end, contains a chordal flat 88 which cooperates withthe chordal flat 82 on sleeve 72 to prevent relative rotational movementbetween the pin and sleeve. The pin is provided adjacent its upper endwith a locator lug 90 which cooperates with a notch 92 designed toreceive the lug recessed into the edge of the sleeve about its upper,greater diameter opening 94.

The upper surface of the pin 84 defines an abutment-receiving portion 96which replicates the abutment-receiving portion 34 of the implantfixture 12 to which the crown 12 will be prepared for application. Thus,there is provided on the upper surface of pin 84 raised hexagonal boss98 though which an opening 100 to an axially disposed, threaded socket102 extends. As shown best in FIG. 3, in the practice of the inventionthe crown 10 is fixedly secured to the pin 84 by means of screw 24. Theconfiguration of the abutment-receiving portion 34, because itreplicates the abutment-receiving portion on the implant fixture 12, iscomplementary to the recessed lower end of the crown abutment 20 andthereby adapted to be readily received in the recess 30 which, in thedescribed embodiment, is hexagonally shaped. Consequently, forcrown-preparation purposes the crown 10 can be readily manipulatedbetween an installed condition in the model and a removed conditiontherefrom by simply sliding the pin 84 with attached crown 10 into andout of the sleeve 72.

To facilitate use of the described implant analog assembly in theworking model, here generally referred to by reference numeral 40', theworking model is formed as two detachable segments, a stationary segment104 and a moveable segment 106. The latter segment 106 is adapted to bealignedly superimposed on the former segment 104 by the cooperationbetween alignment pins 108 fixedly set in segment 106 for reception inalignment holes 110 provided in segment 104.

Construction of the model 40', the sequential steps of which are shownin FIGS. 9, 10, and 11, is distinguished from construction of thepreviously described conventional working model 40 to the extent that,prior to installing the formed impression 52' with attached coping 44and coping screw 48 in the molding tray 54, the pin 84 of the implantanalog assembly 70 is threadedly connected to the threaded end of thecoping screw 48 and thereby securely fastened thereto and the analogsleeve 72 slidably placed upon the anchored pin (FIG. 9). Moldingmaterial, preferably die stone, is then packed into the formedimpression 52 to a level slightly spaced below the inverted upper end ofthe sleeve 72 thereby forming, upon hardening, the movable segment 106of working model 40'. Following solidification of the material formingthe movable segment 106, a parting agent which may be any one or more ofknown types of such agents, such as soaps, waxes or silicones, isapplied to the exposed surface of the segment 106 and of the alignmentpins 108 and another layer of molding material is placed upon thesegment 106, this layer being to a depth sufficient to substantiallycover the alignment pins. (Typically, following application of thesecond layer of molding material, recess-producing forms 112 are placedin the surface of the molding material around the tips of the alignmentpins 108 and the end of the analog pin 84, and the impression tray isinverted placing the second layer on a support surface whereupon anyexcess material which may extend beyond the peripheral edge of the trayis trimmed.) Following hardening of the second layer, the stationarymodel segment 104 is produced wherein, upon removal of the forms 112,recesses 114 are formed on the bottom surface of the segment whereuponthe tips of pins 84 and 108 are accessible to assist in the case of pin84 in releasing the crown-bearing analog pin during the preparationprocedure and, in the case of pins 108, the separation of the respectivemodel segments.

It will be appreciated that, by utilization of implant analog assembliesas described herein, in which the upper crown abutment-receiving surfaceof the pin substantially replicates the corresponding surface of theimplant fixture previously installed in the patient's mouth, repeatedremovals and insertions, as are required by a laboratory technicianduring the course of preparation of the crown can be conducted easilyand rapidly and without danger of damaging the model by erosion orbreakage.

In FIGS. 12 to 29 of the drawings are shown various forms ofabutment-receiving portions 34 and 34a through 34e of implant fixtures12 and 12a through 12e, as well as implant analog assemblies 70 and 70athough 70e having abutment-receiving portions corresponding to those onthe respective implant fixtures. It should be understood that thevarious forms of implant fixture abutment-receiving portions andcorresponding replicated abutment-receiving portions on the analog pinsof the respective implant analog assemblies are intended to berepresentative and not exhaustive of the forms which may be adopted foruse with the invention.

FIGS. 30 and 31 illustrate an embodiment of the invention in which theimplant installation is of the standard abutment and cylinder assemblytype wherein the crown 10' incorporates a core 120 having a cylinder 122integrally formed at the base of the core. Intermediate the cylinder 122and the implant fixture 12 is a separately-formed abutment 124containing a hexagonal recess 126 for reception on thecomplementarily-formed head on the fixture. At its upper end theabutment 124 has a conically-formed recess 128 for receiving theself-centering conical end 129 of the cylinder 122. Connection of theabutment screw 130, whose enlarged head 132 contains a conical shoulder134 adapted to seat on a cooperating shoulder in the bore of abutment124, is provided with an internally threaded bore 138 adapted to receivea cylinder screw 140 for mounting the crown 10 on the implant fixture12.

The implant analog assembly 150 shown in FIG. 31 for use in preparationof the crown 10' incorporates a sleeve 152 which corresponds in shape tothe sleeve 72 of the earlier embodiments, 70 and 70a to 70e. Theassembly pin 154, on the other hand, has an upper portion 156 that isaxially elongated in order to replicate the abutment 124 and has an endface 158, which corresponds in shape to the combined upper ends of theabutment and abutment screw 130, having a threaded bore 160 that permitsreception of the screw 140 for connecting the crown 10' to the pin 154.The structural configuration of the assembly 150 in other respects andits operation is the same as has been described above in connection withthe implant abutment assembly 70.

It will be understood that various changes in the details, materials andarrangements of parts which have been herein described and illustratedin order to explain the nature of the invention, may be made by thoseskilled in the art within the principle and scope of the invention asexpressed in the appended claims.

What is claimed is:
 1. Method of producing a working model forpreparation of a clinical crown for attachment to an implant fixture ina patient, comprising the steps of:attaching a coping to the implantfixture mounted in the mouth of the patient; pouring dental impressionmaterial about the selected region of the mouth of the patient toenclose said coping and the region of the mouth on adjacent sidesthereof to form an impression mold; removing an assemblage of the formedimpression mold with the coping attached from the patient's mouth andconnecting said assemblage to a model molding tray; attaching to theexposed end of said coping an assembled separable analog pin and sleeveassembly; packing molding material in said impression to a predeterminedlevel leaving said analog sleeve exposed; permitting said mold materialto solidify to produce a movable model segment; thereafter, placingadditional molding material in said molding tray to substantially coverthe exposed surface of the solidified movable model segment and saidanalog sleeve; permitting said additional molding material to solidifyto produce a stationary model segment; removing said impression mold andsaid coping; and attaching a clinical crown to the pin element of saidpin and sleeve assembly.
 2. The method according to claim 1 includingthe step, prior to placing said additional molding material, of mountingpositioning pins in the exposed surface of said movable model segmentwith their axes substantially parallel to the axis of said analogassembly.
 3. The method according to claim 2 in which anabutment-receiving portion of the analog pin substantially replicatesthe corresponding portion of the implant fixture in the mouth of thepatient, and including the step of providing said crown with an abutmentsized to be received on said implant fixture, and attaching said crownto said pin employing a fastener connection corresponding to thefastener connection for connecting said crown to said implant fixture.4. The method according to claim 3 including the steps of preparing saidcrown by removing said movable model segment from said stationarysegment by slidingly moving said retention pins from receptor holes insaid stationary segment.
 5. The method according to claim 1 includingthe step, prior to placing said additional molding material, of applyingseparation material to said exposed surface of said solidified movablemodel segment.
 6. The method according to claim 5 including the stepprior to placing said additional molding material, of applyingseparation material to the exposed portions of said positioning pins. 7.The method according to claim 1 in which an abutment-receiving portionof the analog pin substantially replicates the corresponding portion ofthe implant fixture in the mouth of the patient, and including the stepof providing said crown with an abutment sized to be received on saidimplant fixture, and attaching said crown to said pin employing asfastener connection corresponding to the fastener connection forconnecting said crown to said implant fixture.
 8. The method accordingto claim 7 including the steps of preparing said crown by axiallyslidingly removing and returning said analog pin and attached crown toand from said analog sleeve.
 9. A working model for preparation of aclinical crown for attachment to an implant fixture installed in themouth of a patient and containing and internally threaded socket forattachment of a crown, comprising:a stationary segment and a movablesegment, both formed of a solidified molding material; said segmentsbeing connected for separation at a parting line therebetween; an analogsleeve containing an axial bore fixedly secured on said stationarysegment; an analog pin axially slidably received in the bore of saidanalog sleeve, said pin having an external shape adapted for snug,slidable reception in said bore and at its upper end being formed toreplicate a crown abutment-receiving portion of said implant fixture;and cooperable means on said pin and sleeve to prevent relativerotational movement between said pin and sleeve in their assembledcondition.
 10. A working model according to claim 9 in which the crownabutment-receiving portion of said analog pin contains an internallythreaded socket for reception of a screw for attaching a clinical crownfor processing to said pin.
 11. A working model according to claim 10 inwhich said analog sleeve is open at its bottom end and said analog pinhas a lower end extending beyond the bottom end of said analog sleeve;and a recess formed in a lower surface of said stationary segmentbeneath said open bottom end of said analog sleeve, whereby access tosaid lower end of said analog pin is provided.
 12. A working modelaccording to claim 10 including axially tapered positioning pins.attached to said movable segment at longitudinally spaced positionsalong said parting line and correspondingly tapered axial boresextending from said parting line into said stationary segment for snugreleasable reception of said positioning pins.
 13. A working modelaccording to claim 12 including a void in the solidified moldingmaterial of said movable segment, said void being disposed in overlyingrelation to said analog pin to permit access and removal of said analogpin from said analog sleeve.
 14. A working model according to claim 13in which said abutment-receiving portion of said analog pin includesmeans forming a shoulder on said pin to restrict axial movement thereofwithin the bore of said analog sleeve; and means forming a surfacedisposed externally of said bore for reception of the abutment of saidcrown.
 15. A working model according to claim 14 in which saidabutment-receiving surface on said analog pin contains a projectionforming a male element for reception of a female element of a crownabutment.
 16. A working model according to claim 14 in which saidabutment-receiving surface on said analog pin contains a recess forminga female element for reception of a male element of a crown abutment.17. A working model according to claim 10 in which said analog sleevehas an axially tapered bore and said analog pin has a correspondinglytapered external surface for snug, sliding reception in said analogsleeve bore; and cooperating flats formed on said analog sleeve and saidanalog pin to prevent relative rotational movement therebetween.
 18. Alaboratory analog for installation in a working model for preparation ofa clinical crown intended for attachment to an implant fixture installedin the mouth of a patient, comprising:an analog sleeve having anexternal surface configured for locked retention in a body of solidifiedmolding material and an axially-extending bore; an analog pin having anexternal shape for snug, sliding reception to a predetermined extent inthe bore of said analog sleeve; cooperating elements on said analogsleeve and said analog pin to prevent relative rotation therebetween;and an abutment-receiving portion on an exposed, crown-receiving end ofsaid pin, and means in said abutment-receiving portion for attaching aclinical crown thereto, said means substantially replicating a means forattaching said crown to said implant fixture and including an axiallydisposed, internally threaded bore for reception of a crown-attachingscrew, said bore opening at the substantial axial center of saidabutment-receiving portion of said analog pin.
 19. A laboratory analogaccording to claim 18 in which said internal bore of said analog sleeveand said external surface of said analog pins are axially tapered; andsaid rotation-restricting elements include cooperating flats formed onsaid bore and the surface of said pin for mutual engagement when saidanalog pin is installed in said analog sleeve.
 20. A laboratory analogaccording to claim 18 in which said analog sleeve has an open bottom andsaid analog pin has an axial end that protrudes from said sleeve bottomwhen said pin is installed in said sleeve.
 21. A laboratory analogaccording to claim 18 in which said abutment-receiving portion of saidanalog pin includes means forming a shoulder on said pin to restrictaxial movement thereof within the bore of said analog sleeve; and meansforming a surface disposed externally of said bore for reception of theabutment of said crown.
 22. A laboratory analog according to claim 21 inwhich said abutment-receiving surface on said analog pin contains aprojection forming a male element for reception of a female element of acrown abutment.
 23. A laboratory analog for installation in a workingmodel for preparation of a clinical crown intended for attachment to animplant fixture installed in the mouth of a patient, comprising:ananalog pin having an external shape for snug, sliding reception to apredetermined extent in a bore formed in said working model; cooperatingelements on said bore and said analog pin to prevent relative rotationtherebetween; an abutment-receiving portion on an exposed,crown-receiving end of said pin replicating a crown-receiving portion ofsaid installed implant fixture; and means in said abutment-receivingportion for attaching a clinical crown thereto, said means substantiallyreplicating a means for attaching said crown to said implant fixture.24. A laboratory analog according to claim 23 in which saidcrown-attaching means comprises an axially disposed, internally threadedbore for reception of a crown-attaching screw, said bore opening at thesubstantial axial center of said abutment-receiving portion of saidanalog pin.
 25. A laboratory analog according to claim 23 in which saidbore and said external surface of said analog pin are axially tapered;and said rotation-restricting elements include cooperating flats formedon said bore and the surface of said pin for mutual engagement when saidanalog pin is installed in said working model.
 26. A laboratory analogaccording to claim 23 in which said abutment-receiving portion of saidanalog pin includes means forming a shoulder on said pin to restrictaxial movement thereof within the bore of said working model; and meansforming a surface disposed externally of said bore for reception of theabutment of said crown.
 27. A laboratory analog according to claim 26 inwhich said abutment-receiving surface on said analog pin contains aprojection forming a male element for reception of a female element of acrown abutment.
 28. A laboratory analog according to claim 26 in whichsaid abutment-receiving surface on said analog pin contains a recessforming a female element for reception of a male element of a crownabutment.
 29. A laboratory analog for installation in a working modelfor preparation of a clinical crown intended for attachment to animplant fixture installed in the mouth of a patient, comprising:ananalog sleeve having an external surface configured for locked retentionin a body of solidified molding material and an axially-extending bore;an analog pin having an external shape for snug, sliding reception to apredetermined extent in the bore of said analog sleeve; cooperatingelements on said analog sleeve and said analog pin to prevent relativerotation therebetween; and an abutment-receiving portion on an exposed,crown-receiving end of said pin and including means forming a shoulderon said pin to restrict axial movement thereof within the bore of saidanalog sleeve; and means in said abutment-receiving portion forattaching a clinical crown thereto, said means substantially replicatinga means for attaching said crown to said implant fixture and including asurface disposed externally of said bore for reception of the abutmentof said crown, said surface containing a recess forming a female elementfor reception of a male element of a crown abutment.